Porous microstructures have been demonstrated to produce characteristic spectral interference patterns. Porous silicon has been used to produce characteristic spectral interference patterns, and other semiconductors and insulators can be etched to produce characteristic spectral interference patterns. Introduction of analyte into such a porous microstructure produces a shift in the spectral interference pattern. The interference based sensing is largely impervious to less complex optical sensing and detection methods.
As an example, chemical or biomolecule detection can be based on changes in the spectral interference pattern that results from the reflection of white light at the interfaces above (air or solution) and below a porous silicon layer formed in a silicon wafer. Spectral positions of the Fabry-Pérot fringes shift as a function of the refractive index of the material filling the pores. Biomolecule penetration into the pores of porous Si layers, driven either by nonspecific adsorption or by specific binding (to an antibody, for instance) is observed as a shift of the Fabry-Pérot fringes to longer wavelengths. This corresponds to an increase in refractive index of the film as protein displaces aqueous solution in the pores.
The tuning of pore sizes, patterns and distributions in layers, such as porous silicon has been demonstrated. Pores can be tuned to trap a particular analyte, for example based upon pore size or by preparing the pores with a material to bind analyte. Pore size is controlled by an appropriate choice of electrochemical etching conditions. A biosensor can be formed, for example, by tuning pore size to accommodate a biomolecule of interest while keeping the pores small enough to avoid light scattering effects. Etching conditions affect pore size. Conditions that can affect pore size include, for example, the type of material being etched, doping levels (if any), resistivity, etching current density, etc. A wide range of pore sizes and morphologies can be obtained.
Single layer porous microstructures that are based on changes in the spectral interference pattern have been used. Others have used multi-layer porous silicon to achieve biosensors based on optical transduction methods other than wavelength shifts of the interference pattern. For example, Martin-Palma et al. detected binding of polyclonal mouse antibodies to an amine-modified porous Si multilayer by observing a reduction of the intensity of reflected light. Martin-Palma, et al. Microelectronics Journal, 2004, Vol. 35, pp. 45-48. Additionally, Chan et al. formed porous Si multilayer structures such as Bragg mirrors and microcavity resonators and used modulation of the photoluminescence spectra from these structures to distinguish between Gram(−) and Gram(+) bacteria. Chan et al., Journal of the American Chemical Society, 2001, Vol. 123, pp. 11797-11798. The non-interference based sensing methods have difficulty in noisy environments, require more sensitive equipment to achieve comparable sensitivity, and fail to account for common measurement conditions, e.g., signal drift due to thermal fluctuation, changes in sample composition, or degradation of the sample matrix.
Porous silicon films with a distribution of pore diameters in the x-y plane (parallel to the surface of the wafer) have been demonstrated as size-exclusion matrices to perform an on-chip determination of macromolecule dimensions. Karlsson, et al, H. J. Colloid Interface Sci., 2003, 266, 40-47. These films were generated by electrochemically etching Si in aqueous ethanolic HE using an asymmetric electrode configuration. Biomolecules penetrate the film and are detected only in regions where the pores are large enough. A disadvantage of this approach is that determination of protein size requires optical sampling over a relatively large area of the porous Si film.